Ceramic materials offer a wide variety of possible physicochemical characteristics which are controlled by the inherent atomic structure and microstructure of the material. Their refractory nature and chemical inertness allow for the use of these materials in environments where conventional materials such as polymers do not survive. Although the potential for growth in new application areas is very strong, their use is actually limited by processing technology which cannot adequately meet the purity, strength, homogeneity, and microstructure requirements of high performance ceramic applications.
It has been estimated that for a typical process which manufactures a high value added ceramic product, the cost of rejecting materials which do not meet specifications is generally in the range of 25 to 75% of the total manufacturing cost for a particular item. These numbers, which reflect processes which use conventional ceramic powders and processing technology are clearly unacceptable and provide much of the stimulus for the resurgence of interest in the development of new or improved ceramic raw materials based on sol-gel and polymeric precursor chemistry.
Many of the current limitations to the use of ceramics in the high technology areas are a direct consequence of the physicochemical properties of the powders used in their fabrication. The raw material powder's average particle size, particle size distribution, agglomerate levels, and dispersion characteristics combine to limit the macroscopic homogeneity in the ceramic's green and final sintered state. The microstructural homogeneity of the ceramic can be optimized by processing powders where these characteristics are tightly controlled. Monodisperse metal oxide powders derived by sol-gel processes are prime examples of materials in this form.
Monodisperse metal oxide powders offer many processing advantages over conventional ceramic powders. The spherical shape and narrow particle size distribution of the monodispersed powders allow for a tight control over the packing of the powder particles in the green ceramic. Since there are very few agglomerates, the particles pack very uniformly with the residual pore sizes in the green ceramic often being on the order of two particle diameters (&lt;1 micron). If dispersions of the powder are slowly settled, very high green densities are attainable due to the statistical ordering of the particles with the residual pore size often on the order of one particle diameter. This is a crucial advantage since residual porosity or agglomerates on the order of a micron (or greater) in size in a fired structural ceramic can act as sources of cracks or defects in the material and lower its overall performance and strength. In processing conventional ceramic powders, the elimination of residual porosity and flaws resulting from poor powder packing and agglomerates is very difficult. This is typically accomplished by application of pressure during densification of the ceramic and by the addition of sintering aids to control grain growth. The application of pressure during high temperature processing is extremely costly and therefore undesirable. Additions such as sintering aids can adversely affect the high temperature properties of the material and therefore are also undesirable. With monosize powders, such processing steps are not necessary to achieve good densities, microstructural uniformity, and sinterability.
In addition to ensuring a more homogeneous green microstructure, monodisperse powders possess special advantages over conventional powders during the firing of the ceramic. Since the sintering temperature is inversely related to the size of the particles, the submicron diameter of the powder particles results in lowering, often by several hundred degrees, the temperature required for sintering the material. The tightly controlled particle size eliminates the problems of differential sintering that can occur in conventional powders where the very small particles begin to sinter ahead of the larger particles in the compact. This process, which often results in undesirably large grain growth and grain size distributions and porosity, can have disasterous consequences for the performance and properties of the ceramic material.
For monosize powders, sintering of the powder particles is rapid and spatially uniform and results in a homogeneous fine grained microstructure in the sintered article. Being a sol-gel derived material, sintering aids, if required, can be included in the powder cheaply and uniformly during the synthesis stage and the composition of the powder can be tightly controlled during processing. These monosized powders offer a unique opportunity for controlling many of the processing variables which now limit the performance and utilization of ceramic products.
Studies have shown that aging of the metal oxide powders can have a profound effect on the surface area and microporosity of the powder. Through appropriate control over the fabrication procedure it should be possible to develop powders and substrates with controlled micro and macroporosity highly useful as adsorbents, gas membrane and biological support materials, scanning electron microscope (SEM) calibrants, high strength structural ceramics and substrates. Presently, titania is used extensively in the construction of oxygen sensors, electronic components, and pigments.
U.S. Pat. No. 4,543,341 reported a route to monodisperse powders by a sol gel route. This work generated a great amount of excitement due to the resurgence of interest in sol gel chemistry and the large number of advantages these materials possess with respect to conventionally formed ceramic powders. In this prior work, a synthetic procedure for producing monodisperse silica from alkoxides was modified and extended to titania and zirconia.
U.S. Pat. No. 4,543,341 also described the synthesis and characterization of a monodisperse, submicron titania powder possessing a spherical shape, uniform dimensions and low state of aggregation. The powder was prepared by mixing equal volume ethanol solutions of titanium tetraethoxide and water and stirring briefly. It was noted that powder isolation procedures must be initiated within thirty minutes of the onset of precipitation, or hard necked aggregates of the individual powder particles would form. Isolation procedures consisted of centrifuging the freshly prepared powder at low speed followed by decantation of the sol. The powder was washed with ethanol followed by washing with alkaline water to impart a negative charge to the titania particle's surface thereby providing a net repulsive interparticle potential which inhibits flocculation of the powders.